Artikel in Serie "Shells and related Scripting Languages"

Make a Bourne Again Shell Script Log its Output to a File

20. August 2023 in Administration, GNU/Linux, Programmierung.

The Bourne Again Shell script presented in this article demonstrates techniques related to capturing and logging output (standard output and standard error stream) of a script into a log file while also delivering it to the regular output destinations (for example the terminal or whatever the caller has chosen to redirect to). Some questions are addressed:

  • How to „clone“ standard output and standard error stream of a shell script to a single log file?
  • Are the messages from both streams interleaved into a single file in the appropriate order?
  • Can a script perform output cloning for some portions of its procedure while skipping it for others?
  • Are there specific requirements for output that is to be cloned to a file?
  • Are there best practices when a script modifies its own output redirections?

The example provided below has been tested on GNU/Linux using GNU „bash“ version 5 and „tee“ version 8.32 from GNU coreutils. It uses the Bash-only feature of process substitution. An implementation in POSIX Shell (possibly using only POSIX tools) would be more difficult.

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Maintaining Multi-Line „stat“ Formats using Bourne Again Shell

20. August 2023 in GNU/Linux, Programmierung.

The stat command from GNU core utilities features not only a --format FORMAT option but also a --printf FORMAT one, the difference being that the latter allows for backslash escapes such as \n.

This allows for custom per-file report formats containing newlines, for example:

stat --printf 'Name: %n\nSize: %s Bytes\n' /etc/passwd

If the format string becomes more complex, the command line soon becomes unwieldy, such as:

stat --printf 'Name: %n\nOwner ID: %u\nSize: %s Bytes\nLast accessed: %x\n' /etc/passwd

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Print all indented Lines following a non-indented Line

29. April 2024 in GNU/Linux, Programmierung.

Some configuration and output text formats contain sections like the following:

foo:
   value1
   value2
bar:
   value 3

In this article, two scripts are presented that print all consecutive indented lines that follow a non-indented line that matches a search pattern given by a regular expression.

This means, given the single argument foo and the standard input above, the scripts should

  • determine the line that matches foo and
  • print the following two lines, but no other lines.

Also, the indentation of the printed lines should be removed.

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